The present invention relates to a route switching system in a communications network.
In recent years, ATM (Asynchronous Transfer Mode) transmission system has been extensively studied and developed as a hopeful basic technology for the wide area ISDN (Integrated Services Digital Network). The ATM transmission system is a technology dividing information into blocks called cells, each thereof having a fixed length of 53 bytes (5 bytes for header and 48 bytes for information field), to achieve high-speed multiplexed information transmission and exchanging. Transmission speed 2.4 `Gb/s is attained by this system.
The communications network based on such ATM transmission system is expected to have applicability to the subscribers system (urban network) in the wide band ISDN in future. When such communications network is structured, survivability of the network comes to have a deep significance. This is because a network failure produces a great injurious effect on the information-oriented society. As to the technology of structuring a network having the ATM transmission system as its base, great efforts are being made for standardization by international standards committees including CCITT. There are demands for a route switching system free from blocking taking advantage of the ATM transmission system in the communications network.
A conventional route switching system will be described below with reference to FIG. 1, FIG. 2A and FIG. 2B.
FIG. 1 shows an example of structure of a communications network, which is formed of a transmitter 1 and a receiver 2 connected by a first transmission line 6 and a second transmission line 7 with a plurality of repeating nodes 3, 4, and 5 of the same function inserted therein. The transmitter 1 comprises a switching portion 8 and a controlling portion 9. The switching portion 8 is arranged in a cross connect structure of an N.times.N full availability trunk group having a plurality of signal lines, N longitudinal lines and N lateral lines, and performs route switching of input cells 11 under the control of the controlling portion 9 to output the cells 11 to the arbitrarily selected transmission line 6 or 7.
The receiver 2 comprises a switching portion 8a and a controlling portion 9a and, in addition, a buffer 10 connected to the second transmission line 7. The buffer 10 is provided for absorbing a difference in delay caused at the time of route switching between the two routes. For example, while a communication is being held along the first transmission line 6, if the first transmission line 6 is switched to the second transmission line 7 (from the illustrated path "a" to the path "b" and from the path "c" to the path "d") due to a trouble of the first transmission line 6, such as line breakage or increase in traffic, without causing blocking, then, the cells 11 which have been transmitted from the side "a" of the output side of the transmitter 1 come to be output from the side "b" as shown in FIG. 2A.
At this time of route switching, the cells 11 input to the side "c" and the side "d" on the input side of the receiver 2 overlap each other around the switched portion on account of a difference in delay between the first transmission line 6 and the second transmission line 7 as shown in FIG. 2B. More specifically, because the second transmission line 7 introduces a smaller amount of delay than that introduced by the first transmission line 6, the cell transmitted over the second transmission line 7 arrives at the receiver 2 earlier than the cell 11 transmitted over the first transmission line 6 to the receiver 2. When two train of cells are input overlappinq each other, the information (cells) at the overlapping portion t.sub.d cannot be ensured.
Accordingly, it is arranged such that the cells at the overlapping portion t.sub.d are temporarily retained in the buffer 10 and, when the inputting of the cells 11 to the side "c" is completed, the cells in the buffer 10 are read out and input to the switching portion 8a, and thereafter, the cells 11 transmitted over in succession to the side "d" are successively output from the receiver 2 through the buffer 10 and switching portion 8a.
Through such arrangement, at the time of route switching without blocking, the overlapping of the cells 11 can be eliminated and proper transmission can be directed.
In the above described route switching system, the overlapping of the cells 11 is absorbed by the buffer 10. However, in order that the cells at the overlapping portion t.sub.d are retained, a buffer of a large capacity is required. Hence, such a problem arises that the communications apparatus including such buffer 10 becomes larger in size and higher in cost.
Further, since the cells 11 transmitted after the route switching has been made are passed through the buffer 10, a delay time therefor is introduced and, hence, a problem arises that the transmission speed of the whole system becomes lower.